Motor-driven wheel driving apparatus has been proposed to improve driving efficiency in the case of driving a wheel by an electric motor, such as for driving a wheel, e.g. of an electric car where the wheel is directly driven by the electric motor. However, since such a motor-driven wheel driving apparatus requires a large torque in the electric motor, it is necessary to use a large motor with high power. This increases not only the manufacturing cost but the weight of vehicle and accordingly, makes it difficult to ensure sufficient driving performance.
On the other hand, motor-driven wheel driving apparatus (wheel motor) equipped with a reduction gear have been proposed where an electric motor and a planetary reduction gear are arranged within a space inside a wheel. The rotational output of the electric motor is transmitted to the wheel via the planetary reduction gear.
When adopting the planetary reduction gear as a reduction gear, its output shaft to transmit the rotational output of the electric motor to the wheel, via the planetary reduction gear, has to take out its output while coinciding its axial center to that of the motor shaft after once having been divided in the axial direction of the motor-driven wheel driving apparatus. Accordingly, problems are caused in such an apparatus where its structure is difficult to assemble and the strength against inclination of the wheels during running of vehicle is low because of the adoption of a small supporting span of the output shaft.
In order to solve these problems, a known motor-driven wheel driving apparatus is shown in FIG. 8. The motor-driven wheel driving apparatus 51 has an electric motor 54 and reduction gear 55 within the inside of a wheel 53 mounted with a tire 52. The wheel 53 can be driven by the rotational output of the electric motor 54.
The electric motor 54 comprises a stator 57 secured to a side of a case 56 arranged inside the wheel 53. A rotor 58 is arranged opposite to the stator 57. An output shaft 59 is mounted to the rotor 58 to transmit the rotational torque of the rotor 58 to the wheel 53, via the reduction gear 55. The stator 57 and the rotor 58 are secured to a side of the case 56 which is sandwiched by covers 60 and 61 to form the electric motor 54.
One end 59a of the output shaft 59 is integrally formed with a mounting flange 62 which is secured to the wheel 53 via hub bolts 63. This end 59a of the output shaft 59 is rotatably supported by a rolling bearing 64 positioned within a shaft insert aperture 56b in the case 56. The other end of the shaft 59b is also rotatably supported by a rolling bearing 65 positioned within a central recess 60a of the outer cover 60.
The reduction gear 55, contained within the case 56, is formed by a plurality of gears 55a, 55b, 55c and 55d. A first gear 55a is integrally formed with and coaxially arranged at the end of the rotor 58. Second and third gears 55b and 55c are secured on the same supporting shaft 66 and thus rotate simultaneously. The second gear 55b and the first gear 55a mesh with each other. One end 66a of the supporting shaft 66 is rotatably supported by a rolling bearing 67 within a recess 61a of the inner cover 61. Its other end 66b is also rotatably supported by a rolling bearing 68 within a recess 56a of the case 56. The force gear 55d is secured on the output shaft 59 and meshes with the third gear 55c. 
According to such a structure, the output shaft 59 of the electric motor 54 is rotatably supported at opposite ends of the case 56. The output shaft 59 is passed through the aperture of the center of rotation of the fourth gear 55d which is the final stage of the reduction gear 55. Also, the output shaft 59 is passed through the shaft inserting aperture 58a of the rotor 58. Thus, the apparatus can be easily assembled by sequentially fitting the structural parts of the reduction gear 55, the inner cover 61, the structural parts of the electric motor 54, and the outer cover 60 onto the output shaft 59 using it as a reference part. In addition, since the output shaft 59 is supported at substantially opposite ends of the motor-driven wheel driving apparatus 51, it is possible to ensure the supporting span of the output shaft 59. Thus, this obtains a sufficient supporting strength against the inclination of the wheel during running of vehicle (see Japanese Laid-open Patent Publication No. 81436/1995).
In such a motor-driven wheel driving apparatus, while the reduction gear 55 can easily assemble various structural parts, the installation space for the rolling bearings 64 and 65 to support the output shaft 59 is limited since the electric motor 54 requires a large mounting space due to its high rotational output. Accordingly, the loading capacity against the inclination of the wheel 53 during running of the vehicle, against the moment load, becomes insufficient, and thus it requires improvement to the durability of the rolling bearings 64 and 65.